Species‐specific rates of growth and grazing loss among freshwater algae1

Abstract

Recent investigations into the population dynamics of phytoplankton communities have emphasized the variabilities of loss rates rather than growth rates in governing the changes that occur. Many freshwater phytoplankton, however, grow at rates that are measurably less than their maximum physiological capability because of nutrient limitation. At any moment the nutrient most limiting in situ division rates varies from species to species within an assemblage and the extents of limitation change with time. Rates of mortality inflicted by herbivorous zooplankton are likewise species‐specific, but the size of algal particles is a good predictor of whether the particle will be grazed at all. We manipulated grazer abundances and nutrient concentrations in enclosures to examine the effects on algal net growth rates. Grazed species were usually unicellular organisms. Some colonial species showed increased growth rates in the presence of grazers; in all cases these species were limited by N or P in situ. The variability of algal growth rates was much greater than that of measured loss rates due to grazing. Alternate loss rates, primarily sinking, seem to exhibit variability similar to growth rates in comparisons among all species in the assemblage.